Erosive-jet diver tool

An erosive fluid jet tool for underwater operation, comprising:

• (a) means for receiving a fluid under elevated pressure for providing hydraulic power to the tool;
• (b) erosive fluid jet nozzle means connected to the fluid receiving means for providing the working output jet of the tool;
• and
• (c) means for providing a counterthrusting force for balancing the thrust on the tool produced by the erosive fluid jet, the counterthrusting means including a counterthrusting fluid jet nozzle connected to the fluid receiving means and facing oppositely to the erosive nozzle means for providing a counterthrusting jet and open ended shroud means coaxially surrounding the counterthrusting jet nozzle, said shroud means including a mixing tube for substantially coaxially surrounding said counterthrusting jet, wherein the value of the ratio r.sub.t /r.sub.j, the effective cross-sectional radius of said mixing tube, r.sub.t, to the effective cross-sectional radius of said counterthrusting jet nozzle, r.sub.j, is sufficiently large to permit the pressure within said mixing tube at the upstream end of said counterthrusting jet to remain above the vapor pressure during operation, said value being at least about 1.62/.sqroot..SIGMA..sub.o, where .SIGMA..sub.o is the operating cavitation number, whereby, during operation, water surrounding the submerged tool is entrained through the shroud means for providing additional counterthrusting force during operation of the counterthrusting means, the erosive fluid jet nozzle means and the counterthrusting means being constructed so that in excess of 50% of the hydraulic power provided to the tool is provided to the erosive fluid jet nozzle means and whereby, during operation, cavitation is substantially prevented from occurring in the entrained flow around said counterthrusting jet within said mixing tube.